New ideas for package designs using plastic core solder balls

Abstract

Summary form only given. A variety of packaging technologies are widely used in mobile phone applications. One of the major concerns is reduction of the assembly space, and to this end, wafer-level chip-size package (WLCSP) and package-on-package (PoP) methods have been adopted. On the other hand, as the market transitions to LTE smartphones, larger dies and packages are needed to meet the demand for higher performance and functionality. However, for larger packages, due to the difference in the coefficient of thermal expansion (CTE) between the silicon die and substrate, more thermal stress is generated at the solder joint. Therefore, thermal reliability is of great concern. Moreover, in large-size PoP, since the solder connection can be deformed by the package weight, causing a solder bridge to be produced, there is a need to regulate the lower ball-pitch limit. Thus, the ability to control the standoff height is also a key factor in PoP design. Sekisui Chemical Co., Ltd. has developed a unique approach to ensuring both package reliability and standoff control, based on the use of plastic-core solder balls (PCSBs). A PCSB consists of a plastic core, surrounded by successive layers of nickel, copper and tin-silver solder. The use of PCSBs enables high package reliability because the stress generated at solder joints is absorbed by the flexible plastic core. Since the core does not deform under the package weight during multiple reflow, it is possible to maintain a specific standoff height, which is determined by the plastic core diameter. This is a very effective method for maintaining the desired gap between the die and the printed circuit board (PCB). In this study, a new PCSB configuration was developed for obtaining even higher reliability, with a focus on control of the standoff height. The results showed that the new PCSB design led to a much higher package reliability than a solid solder ball series. This made it possible to consider a range of different package designs, including WLCSPs with a size larger than 10 × 10 rum square. It also provides more options for packaging materials, and enables the use of simple structural designs, such as WLCSPs without the need for an under-bump metal (UBM). The combination of larger WLCSPs and a simpler structure would allow the integration of more functions than is possible with other packaging methods. This is also likely to lead to an increase in the number of applications to which WLCSP can be applied. On the other hand, since unlike conventional solder balls, PCSBs allow the standoff height to be controlled without the risk of solder-bridge formation between pads, finer ball pitches can be used in PoP, and the number of 110 pads can be increased without increasing the package size. Thus, this technique can lead to both excellent reliability and a fine ball pitch in packages. It also allows the use of larger WLCSPs without the need for an UBM, and a finer ball pitch in PoP. This makes it feasible to integrate more functions using familiar packaging technologies without a significant increase in capital expenditure.